It seems the world is never quite as simple as one anticipates. It's almost like the more I assume something without actually having prior experience the more likely I am to be wrong. You might be guessing about now that closing the roof damper didn't have the dramatically positive results I anticipated. You'd be right.
Here's some results:
11/20/2013 - high roof temp 96 degrees F, temp rise 33 F, very windy day
11/21/2013 - high roof temp 104F, temp rise 32F, windy day
11/22/2013 - high roof temp 101F, temp rise 28F, calm day
I'm slightly embarrassed to admit I was hoping the temp diffs would get into the high 30s with the roof vent closed. Not even close. The biggest positive difference is that when it is very windy the roof temperatures no longer plummets, like they used to. On Sept 20 a 96 degrees F high was a good result because it was so windy that a big rig truck actually blew over on a local highway. Also a 33F temp rise was very good under those circumstances. I'm sure with the vent open I previously would have been lucky for that temp rise to get to 25F from past experience of windy conditions.
What I did not take into consideration is that a closed roof vent works not only on the front end but also the back end. In summer one is into the air conditioning season. The sun is still up greater than 12 hours. So a closed vent in that season will mostly restrict the exit of hot air percolating under the roof.
In winter it can be just the opposite. I've been monitoring the times at which the roof temperatures cools back down to ambient outside air temps. Usually its between 7 and 8 PM. It doesn't require a open vent for this to happen. The roof just radiates heat it has accumulated to the cold night air, regardless. If I had a monitoring system like Xringer does I could show you. Gotta get one one of these days.
For instance the roof got cold soaked on 11/22/2013. At 7 A.M the internal roof temperature was 10 degrees colder than the outside air temperature. Because the roof vent was closed the warmer ambient air temperature could not warm up the roof and evacuate the cold air that accumulated inside during the night. At 9 A.M. even as the roof was warming the air was warming just as fast. So the roof was still 10 degrees colder than outside air temp. Because the days are so short this time of year there really isn't time for that beginning temperature deficit to be overcome as the day heats up. Soooo, the temperature rise in winter with the vent closed at night and early morning will be lower than it otherwise would be, except on a windy day.
Anyway, that's my excuse for now, and I'm sticking to it.
